Co-doping of ytterbium (Yb) with erbium (Er) has enabled power scaling in the 1550 nm spectral region. However, several issues, such as bottlenecking of the energy transfer, as well as quantum inefficiency, have limited significant power scaling of these co-doped fibers. To overcome this limitation, resonant pumping has been demonstrated, with high efficiency. Yet, this approach requires a 1470-1535 nm pump source and a long length of fiber to overcome bleaching effects. The long fiber length can limit the power scaling for single frequency applications. In addition, 1470-1535 nm pump sources are either fiber-based (via Raman conversion) or diode-based with low brightness. Although these approaches can be beneficial for very high power scaling due to the low quantum defect, the long fiber length can limit the overall efficiency for moderate power applications.
It has been recognized that typical high power Er:Yb co-doped fiber laser amplifiers (EYDFA) suffer from inefficiency at high power levels. This is due, in part, to the bottlenecking process which occurs when the excited Yb ions cannot transfer energy quickly to the Er ions, thus promoting the Yb ions to lase. As disclosed herein, by distributing the pump absorption, a significant improvement in the efficiency of these co-doped fibers systems can be realized by reducing the bottlenecking effect.